The long-term objective of this project is to understand the brain bases of the mental lexicon, which contains memorized words, and the mental grammar, which contains rules that combine lexical forms into larger words, phrases, and sentences. We propose that the memorization and use of words is subserved by temporal-lobe circuits previously implicated in the learning and use of fact knowledge, whereas the acquisition and use of grammatical rules is subserved by frontal/basal-ganglia circuits previously implicated in the learning and expression of motor, perceptual, and cognitive "skills," such as riding a bicycle. Thus we posit that lexicon and grammar are linked to distinct brain systems, each of which is domain-general in that it subserves non-language as well as language domains. This novel view contrasts with the two main competing theoretical frameworks. Although we share the perspective of traditional dual-system theories in positing that lexicon and grammar are subserved by distinct systems, we diverge from these theories where they assume components dedicated (domain-specific) to each of the two capacities. Conversely, while we share with single-system theories the view that the two capacities are subserved by domain-general circuitry, we diverge from them where they link both capacities to a single system with broad anatomic distribution. To distinguish our theory from the other two, we will probe the brain bases of irregular and regular word transformations, in which lexicon and grammar can be contrasted, while other factors are held constant. Irregular forms (e.g., dig-dug) are retrieved from memory, whereas regular forms (e.g., look-looked) require a suffixation rule. We predict, and have found in our preliminary studies, links among irregulars (lexicon), facts, and temporal- lobe circuits, and among regulars (grammar), skills, and frontal/basal-ganglia circuits. Single-system models do not make this set of predictions, and traditional dual-system theories do not predict the links with fact and skill use. Patients with either temporal-lobe or frontal/basal-ganglia damage will be given tasks probing the production and judgment of irregular and regular past tense inflection, plural inflection (mice, bees), and derivational morphology (solemnity, awkwardness), as well as measures of fact and skill use. Our specific aims are to test three hypotheses by probing for double dissociations between irregulars and regulars, and between facts and skills: (1) Lexicon is linked to temporal-lobe circuits, and grammar to frontal/basal-ganglia circuits. (2) These circuits also subserve fact and skill use, respectively. (3) The basal ganglia play a similar role in motor activity and grammatical rule use.